Telemetry is the process of obtaining data remotely by transmitting information from a marine mammal or by storing information for later retrieval. Telemetry includes a number of research approaches, from simple radio tags that allow researchers to relocate a tagged animal to complex data loggers that record data from multiple environmental sensors. Recent advances in this field have led to extraordinary insights into the behavior, ecology, and physiology of marine mammals. It is now possible to obtain data via satellite on the fine-scale behavior of marine mammals from the most remote regions of the world’s oceans. It is also possible, for the first time, to record on videotape what a marine mammal sees as it swims through the water column. These advances allow researchers to investigate how marine mammals use their three-dimensional world and to identify and quantify important physical and biological aspects of their environments. To understand the way in which telemetry has changed our understanding of marine mammals, one need only consult some of the older references on the diving capabilities of marine mammals. These early studies relied almost entirely on anecdotal observations, such as observations of sperm whales (Physeter macrocephalus) tangled in submarine cables. Today, we have a rich understanding of the diving capabilities of marine mammals from direct observations made using telemetry. The field continues to develop rapidly, fueled by continuing advances in technology and miniaturization.
There are two primary approaches to collecting data with telemetry systems. In the first approach, a data logger is attached to a marine mammal, records data for a predetermined period, and then is recovered, allowing researchers to download the information stored in the package. In the second approach, information is transmitted from a marine mammal via radio or acoustic signals.
I. Recoverable Data Loggers
Data loggers record information from a variety of sensors that provide insight into a marine mammal’s behavior, physiology, and environment. The earliest data loggers were simple time-depth recorders that used smoked glass discs rotating past recording needles attached to pressure transducers. These devices were developed in the 1960s by Gerald Kooyman to study the diving behavior of Weddell seals (Leptom/chotes weddellii). To record time in these devices, Kooyman incorporated simple kitchen timers. This ingenuity is typical of the field of marine telemetry, which is too small to support much of its own commercial research and development. Instead, biologists have adopted, modified, and refined technology developed for other purposes.
Modern data loggers are sophisticated digital devices, capable of storing large quantities of information. Data are collected from one or more sensors that measure depth, water temperature, light intensity, or swimming velocity. The sampling interval is set by the researcher and may vary depending on the type of question being asked. The location of a tagged animal can be determined by several methods, but the most common approach is to record light levels and times of dawn and dusk and to back-calculate latitude and longitude after the logger is recovered. It is also possible to record physiological data, such as heart rate and body temperature. Researchers can even record feeding events by transmitting temperature changes in the stomach to external data loggers. To monitor feeding events, a small transmitter, equipped with a temperature sensor, is introduced into the stomach of an animal and transmits data to a data logger mounted on its external surface. Most prey are heterothermic, or cold-blooded, so when they are swallowed, the temperature of the stomach drops abruptly. Eventually the transmitter is passed or regurgitated.
Two recent developments with data loggers have yielded new insights into the behavior of marine mammals. The first is the development of a recoverable logger capable of recording sound—in essence, recording the acoustic environment of the tagged animal. These loggers incorporate hydrophones and recording media such as digital audio recorders or miniature hard drives, as well as other sensor systems. In addition to the vocalizations of the animals themselves and the sound of nearby vessels, the loggers have provided unexpectedly rich data on swimming stroke and heart rate. These devices are being used by researchers to study the effects of anthropogenic noise on the behavior of marine mammals.
The second advance in logger technology is the critter-cam—a recoverable video recorder system. Developed by researchers from the National Geographic Society, crittercams provide a visual record of everything that a marine mammal sees (if aimed forward) or everything in the path of the animal (if aimed backward). Because humans are visual creatures, these video records provide researchers with powerful insights into the underwater lives of marine mammals.
Data loggers have several advantages over other types of telemetry systems. First, because data storage requires considerably less power than data transmission, fewer and/or smaller batteries are required. In turn, this means that recoverable loggers are generally smaller than transmitting tags. Second, the storage of large quantities of data is possible, particularly with modern digital technology. As noted later, transmitting systems limit the quantity of data that can be relayed from the animal to a receiver.
The primary disadvantage of these systems is the need to recover the data loggers to retrieve stored information. The use of data loggers in studies of pinnipeds is fairly straightforward because these animals haul out at predictable times and locations. Researchers studying elephant seals (Mirounga spp.), for example, are able to recover up to 95% of their loggers because of the strong fidelity of these animals to their rookeries. Using data loggers with cetaceans, however, is considerably more challenging, as researchers must first attach the package to a dolphin or whale and then recover the tag after it is jettisoned. One solution to these problems is to attach the loggers with suction cups, fired from a cross bow, and then to recover the buoyant packages after release by homing in on a radio signal emitted by a tag in the package. Several researchers have employed this technique with success, although the logistics of such field work are considerably more complex than the deployment and retrieval of data loggers 011 pinnipeds.
II. Transmitting Systems
Transmitting systems have also undergone a rapid development over the past several decades. The earliest transmitters were omnidirectional radio or acoustic transmitters that allowed researchers to relocate a tagged animal but did not provide information on its behavior or physiology. These simple systems have evolved into sophisticated systems in which large quantities of data can be recorded, compressed, and transmitted.
Some of the earliest radio transmitters used with marine mammals were developed by Bill Watkins and W. E. Schevill from the Woods Hole Oceanographic Institution. In the 1960s, these researchers developed implantable radio tags fired into the blubber of large whales. Similar tags are still used today to study the movements and behavior of baleen whales. Radio tags have also been attached to the dorsal fins of dolphins and porpoises or glued to the fur of pinnipeds by other researchers. When successful, these tags have allowed researchers to follow marine mammals at sea and gain insight into their behavior and short-term movements. This labor-intensive field work requires the use of directional receiving antennae to home in 011 the radio signal produced by the transmitter.
The utility of these simple transmitting systems is limited by several factors. First, because the high-frequency signals emitted by radio tags attenuate rapidly in salt water, it is possible to receive signals only when the transmitter is above the surface. This complicates the tracking of animals because only a few signals are heard at each surfacing. Acoustic signals propagate for much greater distances underwater but often overlap with the hearing range of marine mammals, limiting their applicability. Even under ideal circumstances, radio transmitters have effective ranges of only a few tens of kilometers, so researchers are forced to stay in close proximity to tagged animals. Finally, the large size and cumbersome design of many early radio tags created significant hydrodynamic drag and resulted in the premature detachment of the packages.
Today, researchers have a wide variety of transmitting systems available to them. The most significant advance has been the development of satellite-linked radio transmitters that allow biologists to track the movements and behavior of marine mammals from their offices. The principle underlying these systems is fairly straightforward. Each transmitter emits a stable radio signal to receivers aboard orbiting weather satellites. As a satellite moves across the horizon, the received frequency of the tag changes, due to the Doppler shift, allowing estimation of the position of the transmitter. Each transmission also includes the identity of the transmitter and any associated sensor data. Data are processed and relayed to the user by modem or e-mail. This technological advance obviates the need for researchers to track animals in the field.
Satellite-linked radio transmitters have been coupled with data logging systems to allow the collection of detailed behavioral or environmental data from marine mammals via satellite. This coupling of data logging and transmitting systems has proven to be very successful because it precludes the need to recover the logger package to obtain sensor data. Typical data collected by these systems include depth and swim speed, although in principle any sensor system can be employed.
The advantages of transmitting systems lie primarily in their ability to provide data in real time. Even satellite-linked transmitter systems can provide telemetry data within a few hours, allowing researchers to monitor the movements and behavior of animals in real time from their offices. Simplicity has its virtues, however, and many researchers continue to use conventional radio tags to assist in the relocation of tagged animals in the field. Spending many hours with a particular individual allows for the collection of fine-scale behavioral data. Although it is often possible to use natural features to identify a dolphin or whale, a simple radio tag can greatly facilitate the relocation of a particular individual in the field.
Satellite-linked data loggers are extremely powerful data acquisition systems, but they do have limitations. Their signals can be received only when the transmitter is above the surface and a satellite receiver is overhead. Energy for signal transmission is a significant limitation with current battery technology, although battery life may be conserved by using a salt-water switch, which suppresses transmissions when the tag is submerged. In addition, because the current satellite system limits each transmission to 256 bits, algorithms are required to compress complex data, such as records of individual dive profiles, prior to transmission.
III. Biological Insights
Advances in the field of telemetry have revolutionized our view of marine mammals. As terrestrial observers, we are limited in our ability to study marine mammals and, in the past, have been limited to collecting data from animals at the surface or ashore. While at sea, most marine mammals spend more than 90% of the time submerged, often in remote or harsh environments in which field research is difficult or impossible. Telemetry offers the potential to peer into the lives of whales, dolphins, and seals as they go about their daily activities of feeding, finding mates, and avoiding predators. For the first time, we can ask how a Weddell seal hunts for food under the Antarctic ice, how an elephant seal makes such long dives, or where blue whales go in the winter months. The insights provided by this technology will continue to challenge our thinking about these animals, particularly as new technological developments improve our ability to collect data at sea.
Elephant seals have proven to be particularly amenable to study with telemetry. These are large animals that haul out to breed and molt at predictable times and locations but which spend the majority of the year far from shore. Thus it is possible to equip individual elephant seals with fairly large telemetry packages and be confident that most packages will be recovered. Researchers in both hemispheres have equipped a large number of elephant seals with recoverable data loggers and, more recently, satellite-linked data loggers. From this research, we now know that elephant seals in the North Pacific make two longdistance feeding migrations each year, one after breeding and the second after the molt. For example, adult male elephant seals travel from central California to the Gulf of Alaska on each migration, a distance of more than 10,000 km in each round trip. Individuals appear to return to the same feeding area each year and, once on the feeding grounds, forage almost continuously. While feeding, individual elephant seals dive repeatedly, spending more than 90% of their time at sea submerged, and sometimes diving to depths of more than 1500 m. Such behavior is consistent with what we know of the diet of these animals; elephant seals feed primarily on mesopelagic squid found at depths of 200 to 1000 m. These prolonged and continuous dives have raised many physiological questions, particularly with regard to the oxygen storage capacity of these animals (see later). The prodigious diving behavior of elephant seals has led some biologists to refer to them as mesopelagic mammals.
Elephant seals are not alone in possessing an impressive diving capacity. Sperm whales have been tracked using telemetry to depths of more than 2000 m during dives that may last for more than an hour. Beaked whales (Ziphiidae) are also capable of long, deep dives. Northern bottlenose whales (Hyper-oodon ampullatus), for example, equipped with time-depth recorders attached with suction cups, have made dives to 1500 m and for over an hour in duration. Studies using satellite-linked data loggers attached to smaller whales, such as belugas (Delphinaptems leucas) and narwhals (Monodon monoceros), indicate that these species are also capable of prolonged, deep dives under the Arctic ice.
Studies with crittercams have provided dramatic findings regarding the behavior of marine mammals. Weddell seals have been videotaped flushing prey from crevices in the ice by blowing bubbles, and researchers have watched Hawaiian monk seals (Monachus schauinslandi) sleep and forage on the sea floor. This type of research has particular relevance to conservation because it is believed that monk seals may be endangered, in part, due to conflicts with commercial fisheries. Documenting the availability of prey and the success rate of capture attempts allows us to test such ideas directly for the first time. Backward-mounted crittercams have been used to study the diving behavior of a variety of marine mammals and, in particular, to investigate how whales and seals can make such long dives without exceeding their aerobic capacities. It now appears, for example, that elephant seals and other marine mammals conserve oxygen by gliding extensively during descent. These animals take advantage of the changes in buoyancy brought about by increased pressure at depth and can descend effectively with little extra expenditure of energy or oxygen.
IV. Future Developments
It is difficult to anticipate what surprises the field of telemetry has in store, but it is clear that these techniques will be an integral component of the toolbox of future marine mammal researchers. In particular, it is likely that more sensors will be developed to take advantage of the success of recoverable and satellite-linked data loggers. New sensors may monitor physiological parameters, such as blood oxygen concentration and blubber thickness. Miniaturization and refinement of the crittercam system will allow researchers to ask how deep-diving species, such as elephant seals and sperm whales, find and capture prey at depth. Interest has been expressed in using marine mammals as autonomous oceanographic data collection vehicles. A sample of elephant seals equipped with sensors measuring temperature, salinity, and depth could, for example, provide considerable information on the oceanography of the Southern Ocean. Marine mammals are adept at exploiting fine-scale oceanographic features that concentrate prey, such as frontal systems, and animals instrumented with appropriate sensors could provide considerable information about the location and dynamics of such processes.
New advances in digital technology will undoubtedly result in substantial improvements in our ability to store, transmit, and receive data. Future readers will no doubt find our current suite of data loggers and satellite-based telemetry systems quaint and outdated. Current advances in wireless technology hold great promise for our ability to telemeter data from marine mammals because many current applications in acoustic and video telemetry are limited by bandwidth: the amount of information that can be transmitted from the animal to a receiver. New low-orbit satellite systems could improve our ability to collect more data from a larger sample of animals. So, too, could a network of autonomous underwater data collection systems, similar to those being developed for oceanographic and military purposes, facilitate the collection of data using acoustic telemetry.
Finally, current research efforts are developing attachment techniques that are less invasive and easier to employ, particularly with dolphins, porpoises, and whales. The use of suction cups to attach data loggers to whales for short periods is a good example of this type of innovation. With the advent of improved technology and smaller, more powerful batteries, the size of telemetry packages will also decrease. This will allow the application of this technology to a wider variety of marine mammal species and for longer periods than has been possible to date. The field of telemetry holds great promise to reveal new and exciting insights into the lives of marine mammals.
